It seems to me that a lot of people confuse the spreading out of force/impact (by extending the arms soft-side-down) with actively slapping the ground. Slapping makes no sense; it just adds to the impact force. To my eyes, it appears to be a misperception, by the students, of the ukemi exercise (for example, crossing the arms over the chest, then spreading them to take the fall) . They aggressively fling their arms open and slap the mats, instead of naturally just letting the arms open out. Do that on hard ground, and it's Ouch City.

Well, actually slapping with the arm will subtract (on a vertical fall) from the amount of force the body has to absorb on impact. Think of it a sort of rocket jet that fires momentarily just before the main body hits. On a mat it's OK. I wouldn't recommend it for the street though.